The invention relates to a granular particulate herbicidal composition with characteristics that permit the application effectively to forests and woodlands aerially with minimum drift.
It is customary to formulate herbicides as water-dispersible compositions which can be readily mixed with water and applied by means of a spraying apparatus. An important class of herbicides which can be applied by this means comprises the symmetrical triazinediones, of which hexazinone, 3-cyclohexyl-6-dimethylamino-1-methyl-s-triazine-2,4(1H,3H)-dione, is the most widely used. Formulations of this herbicide are marketed throughout the world as weed-killing compounds under the trade name Velpar.RTM. (trademark of E. I. du Pont de Nemours and Company, Wilmington, Delaware).
One difficulty with aqueous herbicide formulations generally is that the active ingredient often has limited solubility in the water in which it is dispersed under the conditions of temperature at which the spraying is carried out. Heretofore, this has meant that the spray concentrations of many herbicides had to be limited to below their solubility limits in order to avoid crystallization within the spray apparatus and concomitant plugging of the spray nozzle. Nozzle plugging is most likely to take place when a crystal size of 150.mu. in any dimension is reached. Such plugging is of significant economic detriment for the reasons that (1) manpower time must be expended to discontinue spraying operations and remove the plugging material and (2) any area sprayed before the plugging is detected and corrected is likely to be covered inadequately. Hexazinone is one of those compounds which, because of its limited solubility in water, frequently incurs plugging when used at concentrations above its solubility limit at temperatures below about 37.degree. C. (98.6.degree. F.). For this reason, liquid spray herbicidal compositions containing hexazinone have been limited to concentrations of about 2-3% by weight of the active ingredient (hexazinone), unless they are heated to raise the solubility limit. Furthermore there is a tendancy for hexazinone to form insoluble trihydrate crystals in cold water even when the concentration is not saturated.
The application of herbicides in granular particulate form does not incur the disadvantage of aqueous formulations provided they can be effectively applied.
U.S. Pat. No. 4,172,714 discloses a dry compactible composition consisting of large pellets or balls, having a volume in the range of about 0.75 to 2.0 cubic centimeters and containing 5 to 25% by weight of a herbicide which includes hexazinone which is useful for aerial application to woodlands. These pellets or balls are advantageous in that they can be produced by dry compaction, are impact and crush resistant while dry and are highly resistant to disintegration in high humidity atmospheres.
Japanese publication No. 59-33201 discloses granules containing 3 to 16 wt % of antimicrobial agents and insecticides as active ingredients for aerial application which are characterized by (1) a particle size of 10-12 mesh, (2) the number of granules per 1 g is 200-350, (3) the percent of particles in which the weight of 1 particle exceeds 5 mg is 10-30%, (4) the percent of particles in which the weight of 1 particle is 2 to 5 mg os 50-85% and (5) the percent of particles in which the weight of 1 particle is less than 2 mg is 5-20%.
U.S. Pat. No. 3,849,105 discloses a granular pesticide that can be applied aerially with minimized drift. There is however, no disclosure of the characteristics of the granules. The only disclosure is of 16 to 20 mesh particles of the carrier.
Japanese Publication No. 48-1502 discloses an agrichemical granule formulation for aerial spreading by helicopter with a rest angle of 35-50%.
Applications which require the herbicide to be in the form of large pellets or balls or other particulate form cannot be effectively applied aerially because of (1) attrition of the particulates by the equipment used to discharge the particulates aerially, (2) drift of the particulates resulting in an ineffective application, and/or (3) inability to achieve control of vegetation.
Where sufficiently large pellets have been used to eliminate drift from aerial applications ineffective biological control of undesired vegetation has resulted. Large pellets or balls may contain too much active ingredient causing injury to desired species or may take too long to break up to effect control of undesired vegetation or may provide inadequate density of ground coverage. Where pellets have met attrition requirements for aerial application the percent of active ingredient has been sufficiently low to make them less attractive economically for control of large areas, e.g., forest or woodlands. Low active ingredient requires reloading more times because of limited capacities of airplanes compared with high percent active ingredients.